The present invention relates to a circuit for limiting the maximum current to an inductive load such as a motor. More particularly, the present invention provides a switching circuit for disconnecting the voltage supply to a motor when the current through such motor exceeds a predetermined maximum value.
In motor control applications, usually semiconductor switching devices must be selected to protect the worst case of current draws while the motor is stalled. The stall currents could vary as much as three to one (3:1) ratio at the temperature extremes. This requires the selection of semiconductors larger and more expensive than necessary to protect the worst stall current at cold temperatures. Current limit techniques are therefore incorporated in these applications, so the size of the switching devices can be limited to handle the required stall torque. (Stall current is linearly proportional to stall torque). Accordingly, any semiconductor device protecting such motor would have to be large enough and have adequate heat sink capability to handle such current. Typically such a protection scheme is called a linear method of protection where the semiconductor device would be in series with the motor.
In connecting such semiconductor devices with the motor circuit, caution must be utilized less the desired protection not be provided. For example, if the semiconductor device is connected between the voltage source and the battery, typically a sensing resistor must be utilized in the semiconductor circuit to measure the voltage. When in such arrangement the sensing resistor is placed on the voltage source supply side of the motor, a common mode voltage problem results which would require complicated circuitry to detect any overloading. If one of the motor brushes are connected to the motor case, and the sensing resistor is placed on the ground side of the motor supply, the motor case itself must be floated from ground. Accordingly, any short from the ground to the motor case would overload the protecting semiconductors as the current limiting sensing resistor would be by-passed.
Another method of protecting motors from overcurrent is a switching method wherein the inductive load or motor would be switched off when a predetermined current level is reached. Upon the decaying of the current to a lower predetermined value, the supply would again be connected to the motor. The output semiconductor protection devices in series with the motor supply would be either in a saturation or cutoff mode. One problem with such switching method is that upon such occurrence as shorting the semiconductor directly to ground results in the by-passing of the inductive load. Without such inductive load, the protective semiconductor devices would again be operated in a linear mode forced to accommodate the short circuit current. As mentioned above, this would require oversize semiconductor devices for power handling capability.
Accordingly, it is an object of the present invention to provide a current control circuit for an inductive load such as a motor.